A clutch assembly of this type is the subject matter of commonly owned applications Ser. No. 141,150 now U.S. Pat. No. 4,349,090 and Ser. No. 312,129 (now U.S. Pat. No. 4,425,990) filed, respectively, Apr. 17, 1980 and Oct. 16, 1981 by Walter Griesser. According to the disclosures of these pending applications, an externally toothed first (e.g. driving) gear carries several peripherally spaced spring-loaded detent members or rockers which extend radially into an inner peripheral groove of the internally toothed coupling sleeve while the latter is in a decoupling position, i.e. is disengaged from a coaxially adjoining second (e.g. driven) gear having external teeth of the same diameter and pitch. Upon an incipient coupling shift of the sleeve, i.e. an axial displacement thereof toward the second gear, the rockers exert an axial thrust upon a synchro ring which is limitedly rotatable relatively to the first gear and has a clutch surface confronting a similar surface of the second gear across a narrow clearance. As that clearance is eliminated by the thrust, the synchro ring is frictionally coupled with the second gear and is limitedly rotated, relatively to the first gear, about the common gear axis. This relative rotation, in a direction dependent on the speed difference between the two gears, causes each rocker to come to rest against one of two bracketing projections on the synchro ring so as to prevent a radial inward displacement of the rockers by camming engagement with the sleeve until the speed difference between the two gears has been substantially eliminated. It is only then that the sleeve can cam the rockers radially inward to complete the couplilng shift.
The rocker members of the described clutch assembly act as levers whose mechanical advantage amplifiers the exerted gear-shifting force as transmitted to the coacting clutch surfaces of the synchro ring and the second gear. This mechanical advantage enables a significant acceleration of the coupling shift since the times t for completing that shift is given by ##EQU1## where J is the mass moment of inertia of the rotating parts whose speed is to be changed, .DELTA..omega. represents the difference in the rotary speeds of the two gears and k is a proportionality factor. The effective increase in the exerted axial force F.sub.a is particularly useful for the shifting of gears in heavy-duty vehicles, such as trucks or tractors, which in many instances are still being shifted manually.
Certain shortcomings of that prior clutch assembly, however, call for further improvement. Thus, the axial thrust exerted upon the synchro ring in the initial phase of a coupling shift is transmitted via beveled surfaces of the sleeve and the rockers giving rise to additional resistance components that must be overcome. The frictional resistance increases with the leverage and may cause jamming even with relatively small values of the aforementioned mechanical advantage. The completion of the shift requires a radially inward depression of the rockers against the force of their biasing springs which is supplemented by the centrifugal force of the rotating first gear. Moreover, if the confronting clutch surfaces of the synchro ring and the second gear are frustoconical (they could also be formed by interleaved annular friction disks as likewise disclosed in the two prior applications), the combined radial and pivotal motion of the rockers may tend to disalign the two clutch surfaces so as to cause unsymmetrical wear.